6 rubber stamps cost $10.50. Which equation would help determine the cost of 15 rubber stamps?
Explanation: There are several equations that could help determine the cost, each with a slightly different approach. Let $x$ represent the unknown cost of 15 rubber stamps. Since 15 rubber stamps cost $x$ , we have the following proportion: $\dfrac{15}{x}$ We can write the fact that 6 rubber stamps cost $10.50 as a proportion: $\dfrac{6}{\$10.50}$ The cost changes along with the number of rubber stamps purchased, and so the two proportions are equivalent. $\dfrac{15}{x} = \dfrac{6}{\$10.50}$